Mesa Boogie Mark III Mod Question

[kramerxxx]

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These are great amps but it is increasingly difficult to order tubes that are 'hot' enough to get a great clean sound at a low volumes. In the past this seemed to be easier to do but lately I get a matched quad and the idle current measured with a Bias-Rite is way too cold. The gain sounds are a little ratty until you get ridiculously loud. To me, adding a way to bias the power tubes up to the 30ma ranges makes the most sense. My current power tubes are in the teens for idle current.

Who has added a Bia Pot to their Mark III? I'm considering the Mansfieldguitars kit and wanted to ask if anyone has done this to their Mesa Mark amps. Mine is the Simul-Class model so the outer power amp tubes are wired Triode and biased slightly hotter than the inner pair, which are wired Pentode.

Mine is a Blue stripe and I know that the last version, the Green stripe, was wired Pentode/Pentode for more power but this sounds like it was put back to what the original Mark I was which sounds like a better idea for a bias pot being added.

Thoughts? Suggestions?

 

[JOEY B.]

You will need to hook the bias pot to the empty R117 slot near the bottom of the preamp board. I did a bias mod to a couple of Mark III amps using a fixed resistor in this slot. I am assuming that you are using EL-34's in the Class-A sockets. If you have 6L6's in those sockets running in the "teens", something is BAD wrong. The sweet spot that I found was between 32-35 mA for the inner sockets and let the outer sockets be what they are (with the 6L6/EL-34 blend). Any hotter and the lead channel gets a bit mushy sounding with much volume. You can also install a Pentode/Triode switch for the outer sockets. This will not affect the current draw much at all, and is a topic for another thread. Good luck.

 

[kramerxxx]

You will need to hook the bias pot to the empty R117 slot near the bottom of the preamp board. I did a bias mod to a couple of Mark III amps using a fixed resistor in this slot. I am assuming that you are using EL-34's in the Class-A sockets. If you have 6L6's in those sockets running in the "teens", something is BAD wrong. The sweet spot that I found was between 32-35 mA for the inner sockets and let the outer sockets be what they are (with the 6L6/EL-34 blend). Any hotter and the lead channel gets a bit mushy sounding with much volume. You can also install a Pentode/Triode switch for the outer sockets. This will not affect the current draw much at all, and is a topic for another thread. Good luck.

Joey, thanks for the reply. Actually I'm running the amp with 4 6L6s right now. The outer sockets are a little higher but both the inner and outter tubes are lower than you would expect them to be. I think the guy that sold me the tubes didn't understand that Mesa amps are biased cold and just gave me a matched set. Anyway I've heard Mark amps with the bias hotter and I think they really have a great tone, both Clean and Lead. Can you explain what you mean by 'hook the bias pot to the empty R117 slot'? Not sure if you mean a resistor or ???

I think that the amp is healthy as it sounds good loud but it just doesn't have that 'bloom' to the notes that I would expect from it. I have new preamp tubes in it. I understand that Mesa changed the Green stripe to be Pentode for more power and I wouldn't mind trying that too, to have a better clean sound. Did you mod yours to this or was already that way? I'm guessing that this would be just to wire it up like the other tubes sockets, right? It is my combo amp and I use it as a 'grab-and-go'. I'd love to get it close to a Lonestar or Electa Dyne clean sound but that might be impossible.

 

[JOEY B.]

Can you explain what you mean by 'hook the bias pot to the empty R117 slot'? Not sure if you mean a resistor or ???

It is an empty place on the preamp board made for a 1/4 watt resistor. It is wired in parallel with R102 (just to the right when looking from the rear of the amp), which is the bias resistor.

 

[kramerxxx]

Thanks, I didn't know which one the bias resistor was.

take care,
Steve